1. Field of the Invention
The present invention relates to circuits for testing power down reset functions of electronic devices.
2. Description of the Prior Art
Most electronic devices need steady power sources to work normally. If the power source breaks off, or if the voltage of the power source is lower than a rated value of the electronic device, the electronic device cannot work normally. When the power source recovers, a user needs to press a reset button on the electronic device to reset the electronic device. However, this method is unduly inconvenient. Nowadays, most electronic devices comprise circuits or chips that have power down reset functions. If the power source breaks off, or if a voltage of the power source is lower than a rated value, the electronic device cannot work normally. Then when the power source returns to normal, the circuit or chip can reset the electronic device automatically. In general, there is a circuit for testing voltage (“testing circuit”) located between the power source and the circuit or chip that has the power down reset function. If a voltage of the power source is lower than a minimum voltage needed to ensure that the electronic device works normally, the testing circuit sends a low voltage signal to a voltage testing pin of the electronic device, and the electronic device cannot work normally. When the power source returns to normal, the testing circuit sends a reset signal to the voltage testing pin of the electronic device in order to reset the electronic device.
The power down reset function of the electronic device must be fully operational in order to keep the electronic device working normally. Therefore, there is the need for a testing circuit to test the power down reset function of the electronic device. Common testing circuits are integrated in the electronic devices themselves. U.S. Pat. No. 5,450,417, issued on Sep. 12, 1995 and entitled “Circuit for testing power-on-reset circuitry,” provides a testing circuit for determining whether the reset function of the power-on-reset circuitry is valid. The testing circuit and the power-on-reset circuitry are parts of a control circuit that is an integrated circuit. If the reset function of the power-on-reset circuitry is found to be defective, and must be discarded, because the power-on-reset circuitry is part of the control circuit, and the control circuit is an integrated circuit, the control circuit must be discarded too. Then because the testing circuit is part of the control circuit, this necessarily means that the perfectly good testing circuit must be discarded too, which is wasteful. In addition, the testing circuit makes the control circuit more complicated.
Therefore, there is a need for an inexpensive, independent and simple testing circuit that can test whether the power down reset function of an electronic device is valid.